This invention relates generally to gas turbine engines and, more particularly, to apparatus for controlling thrust in engines of the gas turbofan variety.
Aircraft manufacturers and operators have expressed a strong interest in a full throttle control for gas turbine engine power plants, a control in which takeoff power, for example, is associated with the same throttle position regardless of altitude and ambient temperature. Presently, power management control for large high bypass turbofan engines is not an easy task for the pilot since takeoff power, for example, is reached at varying throttle positions depending primarily upon the ambient air temperature, and also aircraft Mach number and altitude.
In practice, gas turbine engines are characterized as possessing a given flat thrust rating. By this it is meant that the engine manufacturer gurantees that the engine will produce a given amount of thrust (called a "rated thrust") over a range of ambient inlet temperatures up to a maximum flat thrust rating point provided, of course, that fan speed or engine pressure ratio is set by the pilot to rated values supplied by rating charts available in the cockpit. When the engine inlet temperature exceeds this predetermined flat thrust rating point, thrust must be reduced in order to protect against turbine overtemperature since, with a given thrust, turbine inlet temperature increases with ambient engine inlet temperature. In fact, the power sensitivity of the engine to throttle position is highly variable depending upon the inlet air temperature to the engine. For example, on a hot day the pilot typcially advances the throttle to its maximum angular position in order to obtain rated takeoff thrust. On a cold day, it is probable that in order to obtain the same value of rated takeoff thrust the power lever would only have to be advanced half as far, and any further throttle advance could result in excessive engine corrected rotational speed and potential overstress of engine components. Thus, the power lever position must be managed manually by the pilot to obtain a given engine thrust level as environmental conditions change in order not to overstress these engine components. The problem is compounded by changes in airport altitude since the engine thrust potential of a gas turbine engine tends to decline in direct proportion to the density of the atmosphere. It would be desirable from the pilot's point of view for particular engine power ratings to be located at fixed positions on the power lever angle scale regardless of changes in inlet temperature, altitude and aircraft Mach number in order to reduce the pilot's workload which currently requires these frequent throttle adjustments.
Past efforts at providing a solution to this problem for gas turbine engines of the high bypass turbofan variety have utilized electrical trimmer controls which receive an input signal proportional to corrected fan rotational speed. Before discussing the shortcomings of the prior art approaches, it appears appropriate at this point to discuss the difference between the physical rotational speed of turbomachinery and corrected rotational speed since both terms will be utilized in the following discussions and claims. Physical rotational speed is the absolute speed of rotation with which most people are familiar, a rotational velocity usually expressed in, for example, feet per second (or meters per second) with respect to a stationary reference frame. Corrected speed is a mathematical representation of turbomachinery performance which is used for correlation purposes, a representation which relates the effects of speed variations and the effects of temperature variations on turbomachinery performance. In other words, a turbomachine's corrected speed, rather than its physical speed, is the indicator of its performance. This corrected speed is defined as being equal to the physical speed divided by a factor representing the variation in temperature from standard day conditions. Expressed as an equation, corrected speed has traditionally been defined as follows: ##EQU1##
The prior art electrical trimmer controls using corrected fan speed have solved the problem, but at relatively high cost and at some sacrifice in engine reliability. Furthermore, although corrected fan speed by itself is an accurate indicator of thrust in a gas turbofan engine when the engine is operating at a stabilized condition, it is not particularly useful during the starting and acceleration of the engine due to the high inertia of the fan and, therefore, rotational acceleration lag with respect to the independently driven core engine. Thus, during starting and acceleration, core engine speed is the more useful parameter and, when selected as a thrust parameter, it allows for the design of a simple, low cost, reliable gas turbine engine thrust control. However, until the present invention, the economic advantages of utilizing core engine speed instead of fan speed as a thrust indicator could not compensate for the deficiencies in the correlation between thrust and corrected core speed for a gas turbofan engine. The reasons are that core engine speed was corrected to the core compressor inlet temperature (i.e., the numerator of the above .theta. correction factor was core compressor inlet temperature rather than engine inlet temperature) and the square root power of .theta. did not adequately correlate corrected speed (RPM) with flat-rated thrust. Also, water ingestion in the form of rain can disrupt such a correlation since some of the temperature rise associated with the fan is utilized in turning the ingested water to steam and the measured temperature at the core engine inlet is not indicative of the work done by the fan. Since core engine speed is necessary for engine starting and transient control, and since it can be correlated with fan speed during steady-state operation, a simple and less expensive core engine speed control can be developed in lieu of a fan speed control which provides most of the operational advantages of a fan speed control at lower cost.